Flame control in the flame-holding region

ABSTRACT

A combustion system includes a fuel nozzle, a charge source, a discharge electrode, and a voltage supply coupled to the charge source and discharge electrode. The charge source is configured to apply a polarized charge to a flame supported by the nozzle, and the discharge electrode is configured to attract a flame-front portion of the flame to hold the flame for flame stability. The discharge electrode can be toroidal in shape, positioned coaxially with the nozzle downstream from the nozzle. The voltage supply is configured to hold the charge source at a charge potential and the discharge electrode at the discharge potential. The nozzle can be configured to apply the polarized charge to a fuel stream emitted by the nozzle, whereafter the charge is passed to the flame upon combustion of the fuel.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority benefit from U.S. ProvisionalPatent Application No. 61/803,080, entitled “FLAME CONTROL IN THEFLAME-HOLDING REGION”, filed Mar. 18, 2013; which, to the extent notinconsistent with the disclosure herein, is incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to combustion systems, and moreparticularly, to methods and devices used to affect flame shape forimproving flame stabilization.

BACKGROUND

Combustion systems are employed in a vast number of applications, inindustry, commerce, and private residences. Tightening governmentregulations, increasing costs of fuel, and public opinion all contributeto a continual pressure to reduce emissions and improve efficiency ofsuch combustion systems.

Flame stability is an important element in most combustion systems. Aflame can be said to be stable when flame propagation speed, i.e., therate at which a flame front travels upstream in a flow of fuel, and thevelocity of the fuel flow are in equilibrium. Some combustion systemsemploy fuel nozzles from which fuel is ejected at velocities that exceedthe flame propagation speed for the particular fuel and conditions, andin which the velocity does not drop below the propagation speed beforethe fuel stream is too dilute to support combustion. Withoutflame-holding structures designed for the purpose, such systems couldnot support a stable flame. The flame front would be swept downstreamand extinguished. A typical flame holder includes a bluff body, aV-gutter, or some other form of flow blockage that is positioned at theperiphery of the fuel stream. The obstruction of the flame holder causesturbulence in the aerodynamic wake of the flame holder, includingvortices in which hot gases from the combustion reaction arerecirculated and mixed with uncombusted reactants, which then ignite,maintaining a continuous ignition position in the lee of the flameholder, thus sustaining a continuous combustion reaction, and a stableflame.

SUMMARY

According to an embodiment, a combustion system is provided with amechanism for holding a flame at a selected location. The combustionsystem includes a fuel nozzle, a charge source, a discharge electrode,and a voltage supply that is coupled to the charge source and dischargeelectrode. The charge source is configured to apply a polarized chargeto a flame supported by the nozzle, and the discharge electrode isconfigured to attract a flame-front portion of the flame to hold theflame for flame stability. The discharge electrode can be toroidal inshape, positioned coaxially with the nozzle downstream from the nozzle.The voltage supply is configured to hold the charge source at a chargepotential and the discharge electrode at the discharge potential. Thenozzle can be configured to apply the polarized charge to a fuel streamemitted by the nozzle, whereafter the charge is passed to the flame uponcombustion of the fuel. The discharge potential is preferably at acircuit ground potential or at a polarity that is opposite a polarity ofthe polarized charge. Because of the electrical difference between thecharge potential and the discharge potential, a flame-front portion ofthe flame is attracted toward the discharge electrode, which serves tohold the flame at a location near the discharge electrode.

In some embodiments in which the discharge electrode is toroidal inshape, the electrode is positioned concentric with a longitudinal axisof the nozzle. During operation, the flame front is attracted to thedischarge electrode at a multitude of points on the surface of theelectrode surrounding the flame.

According to an embodiment, the flame front makes intermittent orcontinuous contact with the discharge electrode.

According to an embodiment, the discharge electrode is one of aplurality of electrodes arranged in radial symmetry around the nozzle.Each of the plurality of electrodes is configured to be held at thedischarge potential. According to an embodiment, the voltage supply isconfigured to selectively apply the discharge potential to some of theplurality of electrodes, while decoupling others of the plurality ofelectrodes from the charge/discharge circuit. In this way, a positionand size of the flame can be at least influenced, if not fullycontrolled, by selection of the positions in which the electrodes arearranged, and selection of the electrodes to be held at the dischargepotential and those to be decoupled from the charge/discharge circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present disclosure are described by wayof example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. Unless indicated asrepresenting the background art, the figures represent aspects of thedisclosure.

FIG. 1 illustrates regions that may be identified in a flame within acombustion volume, according to an embodiment of the present disclosure.

FIGS. 2-4 are schematic diagrams of combustion systems, according torespective embodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings, whichare not necessarily to scale or proportion, similar reference characterstypically identify similar components, unless context dictatesotherwise. The illustrative embodiments described in the detaileddescription and drawings are not intended to limit the scope of theclaims. Other embodiments and/or other changes can be made withoutdeparting from the spirit or scope of the present disclosure.

In the embodiments disclosed below, various electrodes are described asbeing configured to apply a charge to a flame, while other electrodesare configured to apply electrical energy to the flame, or to dischargethe flame, etc. An element can be said to be charged when it has eithera surplus of electrons, in which case it is negatively charged, or ashortage of electrons, in which case it is positively charged. Thus, anelectrical charge inherently includes a polarity. The element can bedischarged by enabling electrons to flow to or from the element until anequilibrium is reached. This equilibrium can be an absolute equilibrium,in which each of the atoms that comprise the element has its nominalcomplement of electrons, or, more commonly, a relative equilibrium, inwhich electrons are permitted to flow freely between two or moreelements until each element is at a same electrical potential. It willbe recognized, therefore, that for the most part, a charge is a relativevalue that is characterized by a difference in electrical potentialbetween two elements, and that the absolute polarity of an element islargely ignored in favor of the relative polarity. In other words, evenif each of a pair of elements has a shortage of electrons and is thuspositively charged, in an absolute sense, if they are not at the sameelectrical potential, one will be negatively charged, relative to theother. If these elements are placed in electrical contact, electronswill flow from the “negatively” charged element to the other (and byconvention current will flow from the more “positively” charged elementto the other), until the difference in potentials is discharged.

With respect to the present disclosure, although a flame can bedescribed as being charged via a charge source or electrode, anddischarged via a discharge electrode, it can be just as correct todescribe the process oppositely, because each electrode acts todischarge the flame with respect to a potential difference between therespective electrode and the flame, and in the process—inasmuch as theelectrodes are held at different potentials—acts to charge the flame,with respect to a potential difference between the opposite electrodeand the flame.

In view of the discussion above, where the specification or claims referto charge and discharge, these terms are interchangeable. Furthermore,because an actual physical attraction can be formed between elementswithout resulting in a discharge of a difference in potentials—the wellknown phenomenon of static attraction being an example—some desiredresults can be obtained without an actual discharge occurring.

Various burner systems are disclosed herein as embodiments. Manyelements are omitted from the embodiments described, particularly wheresuch elements are not necessary for an understanding of the principlesdisclosed. In practice, these and other embodiments typically includemore extensive combustion systems used in industry and commerce as partsof, for example, electrical power generation, boilers, refineries,smelters, foundries, commercial and residential HVAC systems, etc.

FIG. 1 is a schematic diagram of a portion of a burner 100 including aburner nozzle 106 configured to emit a fuel stream 102 along alongitudinal axis A of the nozzle and to support a flame 104, accordingto various embodiments. Relative particle velocity within the flame 104is represented by arrows V, of lengths corresponding to relativevelocity. For the purposes of the present disclosure, a flame can bedivided into three general regions or portions. The first region R₁,closest to the nozzle 106, is a flame-holding region. Adjacent to, anddownstream from the flame-holding region R₁ is the second region R₂, amomentum-dominated fluid dynamics region, and furthest downstream, thethird region R₃ is a buoyancy-dominated fluid dynamics region.

The term flame particle refers primarily to gaseous atoms and/ormolecules (e.g., gaseous fuel, fuel vapor, nitrogen, oxygen, argon, andreaction intermediates) that comprise the fluid within a flame, as wellas the small solid (e.g., soot and/or ash) particles that may beentrained within the flame.

A flame front 108 of the flame 104 is located in the flame-holdingregion R₁. As fuel flows from the nozzle 106 in a downstream directionin the fuel stream 102, the flame front 108 is continually movingupstream. The velocity of the fuel stream 102 is a function of a numberof factors, including the geometry of the nozzle 106 and the pressure ofthe fuel within the nozzle 106. As the fuel stream 102 moves downstream,it slows as the flow diverges and entrains air from the surroundingatmosphere. Meanwhile, the flame propagation rate, i.e., the speed atwhich the flame front 108 moves upstream in the flow of fuel, dependsupon factors that include the type of fuel, the amount of entrainedoxygen, ambient temperature, etc. When the flame propagation rate andthe fuel stream velocity are at equilibrium, the flame 104 remainssubstantially stationary relative to the nozzle 106, and the flame 104is said to be stable.

Within the momentum-dominated fluid dynamics region R₂ of the flame 104,the velocity and vector of flame particles within the flame 104 aresubstantially determined by the velocity and vector associated with thefuel stream 102. In this region, the velocity of the flame particles issufficiently high that other common factors, including buoyancy, havelittle influence on their vectors. However, as the flame particles movedownstream, they lose velocity, and the buoyancy of the flame 104,relative to the cooler and denser surrounding gases, tends to push theflame upward. As the flame particles move further downstream andcontinue to lose velocity, the direction of movement is increasinglydominated by flame buoyancy.

The shape of the flame 104, and the relative dimensions of the threeregions R₁-R₃, can vary significantly, according to many factors. Forexample, in some cases, the buoyancy-dominated fluid dynamics region R₃is nonexistent, or very nearly so, as in, e.g., some welding torchflames. In these types of flames, the fuel is substantially consumedbefore the velocity has dropped to a level where buoyancy can exert asignificant influence. In other cases, the momentum-dominated fluiddynamics region R₂ is substantially nonexistent, as in, for example, thecase of a candle flame or other flame in which little or no velocity isimposed on the flame 104 by the fuel, so that the velocity and vector ofthe flame particles are entirely controlled by other factors, includingbuoyancy.

A flame can stabilize very close to the nozzle 106 or some distancedownstream, depending upon the various factors mentioned above. However,in some systems, the initial fuel stream velocity is sufficiently high,that by the time the stream has slowed to below the flame frontpropagation speed, the fuel has entrained enough air that it is toodilute, and is no longer flammable. Typically, such systems employ sometype of flame holder configured to generate turbulence, as discussed inthe background above.

As described herein, the inventors have recognized that the interactionof the flame 104 with one or more electrodes can affect flame stability.In particular, it is noted that the interaction of a charged flame (orcharged fuel stream) with a flame holding electrode can maintaincombustion, even when current flow is controlled to prevent sparkdischarge. It is further noted that actuation of one or more electrodes(e.g., mechanically or otherwise causing selective positioning of anelectrode) can also interact with combustion fluid flow adjacent theretoand thereby affect flame stability. Moreover, the inventors noteinteraction between electrode actuation and electrode voltage (and/orcurrent flow).

Controlling an electrical and/or fluid dynamic interaction between theflame 104 and/or fuel flow 102 can thus affect flame stability andproduce other desired effects. The other desired effects can includereducing pollutant emissions, reducing sooting, increasing blackbodyemissivity of the flame, increasing maximum heat output, and/orexpanding turn-down (reducing minimum heat output) of the flame 104.

FIG. 2 shows a schematic view of a combustion system 200, according toan embodiment. The system 200 includes a toroidal electrode 202 and avoltage supply 204. The nozzle 106 is configured to act as a chargeelectrode, i.e., a portion of the nozzle 106, preferably at least theupper rim, is electrically conductive, and is electrically coupled tothe voltage supply 204. The toroidal electrode 202 is centered on thelongitudinal axis A of the nozzle 106 in a position that falls withinthe flame-holding region R₁ of a flame 104 supported by the nozzle 106,and is coupled to circuit ground 206.

During operation, a charge is imparted to the fuel stream 102 as itleaves the nozzle 106. The charged fuel stream 102 is ignitedsubstantially as it passes through the toroidal electrode 202, and itscharge is passed to the flame 104. The inventors have discovered that inan electrically charged flame, the flame seeks a discharge opportunity,and moves toward such an opportunity, to the extent possible. In thepresent case, the flame front 108 of the charged flame 104 is attractedto the grounded toroidal electrode 202, where a portion of the chargecan be released. Consequently, the flame front 108 remains in contactwith the toroidal electrode 202 even when the velocity of the fuelstream 102 far exceeds a normal flame propagation speed for the fuel andconditions present. The inventors have found that the flame front 108does not usually contact the toroidal electrode 202 around its entirecircumference, especially at higher fuel stream velocities, but insteadwill maintain contact at one or two points on the electrode 202, whichpoints of contact move from place to place on the electrode duringoperation.

Because the toroidal electrode 202 is larger in diameter than the conedefined by the diverging fuel stream 102, the electrode 202 does notintroduce a significant amount of turbulence or drag into the fuelstream 102 or flame 104, which reduces or eliminates the energy andefficiency losses normally associated with flame holders in conventionalcombustion systems.

In the embodiment shown, the toroidal electrode 202 is coupled tocircuit ground 206. This can be advantageous, because it means that thecharge electrode—in this case, the nozzle 106—can apply either apositive or negative charge to the fuel stream 102, and in either case,the flame 104 will seek to discharge on the toroidal electrode.According to other embodiments, the toroidal electrode 202 is coupled tothe voltage supply 204, which is configured to apply a voltage potentialhaving an absolute value greater than zero to the electrode 202. Wherethe voltage applied to the toroidal electrode 202 is greater than zero,the applied voltage preferably has a polarity that is opposite apolarity of the charge applied to the flame 104. One possible advantageof applying voltages greater than zero to both the nozzle 106 and thetoroidal electrode 202 is that a greater voltage difference can beachieved for a given available maximum absolute value.

The appropriate magnitude of the voltage difference between a chargeelectrode and a discharge electrode will vary according to a number offactors, including, for example, the velocity of the fuel stream 102relative to the normal flame propagation speed, the difference indiameters of the toroidal electrode 202 and the cone of the fuel stream102 at the point where it passes through the toroidal electrode 202, theambient temperature and humidity, the availability of oxygen, etc.

According to various embodiments, the voltages applied to the nozzle 106and/or the toroidal electrode 202 can have a positive polarity, anegative polarity, or can alternate, in a regular time-based signal. Inparticular, the inventors note somewhat enhanced effects are produced bya relatively positively charged flame interacting with a relativelynegative voltage (e.g., grounded) toroidal electrode 202 compared toinverted DC voltages. Similarly, the inventors note that increasing aportion of a regular time-based signal (e.g., a voltage-biased ACsignal) during which the flame is relatively positively charged relativeto the toroidal electrode (compared to an inverted polarityrelationship) tends to pull the base of the flame downward toward thetoroidal electrode 202. Accordingly, varying a bias voltage or atemporal duty cycle of a time-varying toroidal electrode voltage and/orcharge electrode signal can be used to select a flame base positionacross a range of fuel flow rates.

Interestingly, the inventors note that at least under some conditions,pulling the base of the flame downward simultaneously moves the tip ofthe flame upward. Accordingly, controlling voltage/current relationshipsand/or toroidal electrode 202 geometry can be used to control flamelength.

Potential differences investigated by the inventors range from about 10kV to about 80 kV. Effects were found to be a function of fuel pressure(and hence velocity) and to be relatively independent of burner scale.

According to an embodiment, the voltage supply 204 is configured to varythe voltage difference between the nozzle 106 and the toroidal electrode202 as conditions within the combustion volume change. According toanother embodiment, the voltage supply 204 is configured to release theflame front 108 from the toroidal electrode 202 under selectedconditions. For example, in the case where the velocity of the fuelstream 102 is reduced, the flame 104 may be capable of stabilizingwithout a flame holder. In another embodiment, the fuel composition andfuel stream velocity are selected to be such that without a flameholder, the flame 104 will stabilize some distance from the nozzle 106,so that the position of the flame 104 is selectable between the toroidalelectrode 202 and the more distant location.

The flame front 108 can be released from the toroidal electrode 202, forexample, by removing the voltage signal from the nozzle 106 so that nocharge is applied to the fuel stream 102, or by decoupling the toroidalelectrode 202 from ground so that there is no discharge path.

FIG. 3 shows a schematic view of a combustion system 300, according toan embodiment. The combustion system 300 is substantially similar to thesystem 200 of FIG. 2, except that the toroidal electrode 202 of thesystem 300 is movable, by operation of an actuator 304. The toroidalelectrode 202 is shown coupled to the voltage supply 204 rather than toground 206, as shown in FIG. 2, but as previously indicated, theelectrical circuit can be arranged in any of a number of configurations,only some of which are shown or described herein. Furthermore, whereboth the voltage supply 204 and the toroidal electrode 202 are coupledto circuit ground 206, they can be considered to be coupled together, aswell.

In the embodiment shown, the actuator 304 is configured to rotate thetoroidal electrode 202 about an axis that lies transverse to thelongitudinal axis A of the nozzle 106. By changing the angular positionof the toroidal electrode 202 relative to the axis A, the apparentaperture size of the electrode can be reduced. According to anembodiment, the actuator is configured to change the angular position ofthe toroidal electrode 202 when conditions within the combustion volumeare such that the flame 104 cannot hold to the toroidal electrode 202 atits normal distance from the axis A. For example, in some cases, whenthe flow rate of the fuel stream 102 is reduced, the diameter of thefuel cone can also reduce, increasing the distance that the flame 104must bridge to contact the electrode 202.

Angular adjustment of the toroidal electrode 202 enables an increasedturndown ratio, as compared to the combustion system 200 of FIG. 2.

According to another embodiment, the actuator 304 is configured to movethe toroidal electrode 202 axially along the longitudinal axis A.Accordingly, when the fuel flow rate is reduced, thereby reducing thediameter of the cone of the fuel stream 102, the actuator 304 isconfigured to move the toroidal electrode 202 in a downstream direction,i.e., away from the nozzle 106, to a position where the diameter of thecone of the fuel stream 102 is once again only slightly smaller than theinner diameter of the toroidal electrode 202. Conversely, as fuel flowincreases, the actuator 304 is configured to move the toroidal electrode202 in an upstream direction to prevent the fuel stream 102 fromimpinging on the electrode 202 and producing undesirable turbulence.

FIG. 4 is a schematic view of a combustion system 400, according to anembodiment. The system 400 includes a plurality of electrodes 402arranged in radial symmetry around the longitudinal axis A of thenozzle. Each of the electrodes 402 is coupled to the voltage supply 204and configured to be held at a discharge potential, relative to a chargeapplied to the flame 104 via the nozzle 106. The discharge potential canbe ground potential, or an absolute value greater than zero and having apolarity opposite that of the charge applied to the flame 104. Theelectrodes 402 can have any appropriate shape, such as, for example,cylindrical or rectangular rods or posts, with or without rounded,conical or sharp tips, amongst others. The position of the electrodes402 along the axis A, i.e., the distance downstream from the nozzle 106,is a design choice that will be based on a number of factors, including,for example, air flow rate, fuel flow rate, type of fuel, differentatmospheric conditions in and around the combustion volume, etc.

In operation, a charge is applied to the flame 104, as previouslydescribed, which then seeks to discharge via one or more of theplurality of electrodes 402. The inventors have found that typically,the flame 104 will hold to one of the plurality of electrodes 402 at atime, but will jump from one to another in an apparently random manner.Of course, if one of the electrodes 402 is decoupled from its connectionto the voltage supply 204 or ground, the flame 104 will not hold to thatelectrode 402. Thus, by selectively decoupling one or more of theplurality of electrodes 402, the location at which the flame 104 holdscan be influenced or selected.

According to an embodiment, the plurality of electrodes 402 can bearranged at various distances radially from the axis A, such as inconcentric circles, for example. Selected groups of the electrodes 402can be energized, according to the particular requirements of themoment. For example, when fuel flow is reduced, an inner ring ofelectrodes 402 can be energized in order to accommodate a flame 104 witha relatively small diameter. Conversely, when fuel flow is increased,rings of electrodes 402 positioned further from the axis A can beenergized, at a distance consistent with the reach of the flame 104.

According to another embodiment, the plurality of electrodes 402 arearranged at various distances axially, downstream from the nozzle. Forexample, some of the electrodes 402 can be positioned near or within themomentum-dominated fluid dynamics region R₂. When the more distantelectrodes 402 are energized, the flame 104 is drawn outward towardthose electrodes 402. This can result in an increased flamediameter—when the additional electrodes 402 are evenly spaced around theflame 104, and are simultaneously activated—or can cause the flame toshift in the direction of one or another electrode 402 that isseparately energized.

In addition to the discharge electrodes 402 disclosed herein, variousother electrode shapes and configurations can be employed, particularlyfor the purpose of holding a flame in a combustion system. For example,a linear conductive body can be positioned extending near to, or throughthe flame 104, in order to stabilize the flame 104.

According to some embodiments, at a higher fuel flow rate in combustionvolume it may not be desired that the flame 104 contact heat transfersurfaces. Accordingly, the flame 104 can be contained at a particular,predefined location, by selection of a particular discharge electrode402, for example, to allow for a high turn-down ratio and a large rangeof fuel flow to provide a high rate to the combustion reaction whilealso preventing flame 104 from greatly increasing in length.

The flame 104 can be held at a certain location. This location can bechanged to move the flame 104 to the left or to the right, up or down,to change flame-holding characteristics without difficulty and to enabledefining the borders of momentum-dominated fluid dynamics region R₂. Theshape of the flame 104 can be affected in the momentum-dominated fluiddynamics region R₂ independently of other actions, such as holding theflame 104, in the flame-holding region R₁.

High voltages or ground can be employed to hold the flame 104 byapplying a charge to the flame 104. The ability to apply a charge to theflame 104 may require the use of charge injection. In order to get acharge injection, high voltage is required to create a field curvatureover an electrode employed for charge injection. For this, an ionizercan be used. Ionizers are typically operated at voltages having anabsolute value of greater than 1000VDC or VAC. According to someembodiments, the ionizer is positioned some distance from the flame 104,and is used to ionize the air feeding the flame 104 with a relativelyhigh ion density, such that a significant volume of ions are entrainedby the flame. The relatively high ion density in the flame 104 theninteracts with ground or relatively low voltages on the electrodes 402.

According to an embodiment, resistance is added to the circuit of one ormore of the plurality of electrodes 402 to modify the voltage at whichthe electrodes 402 are placed. This voltage can be calculated toregulate voltages on the electrodes 402 to allow differences in the wayin which and/or the location where the flame 104 is held.

While various aspects and embodiments have been disclosed, other aspectsand embodiments may be contemplated. The various aspects and embodimentsdisclosed here are for purposes of illustration and are not intended tobe limiting, with the true scope and spirit being indicated by thefollowing claims.

What is claimed is:
 1. A combustion system, comprising: a fuel nozzleconfigured to emit a fuel stream; a charge source configured to apply apolarized charge to a flame supported by the nozzle; and a firstdischarge electrode positioned and configured to attract a flame-frontportion of the flame when energized at a discharge potential.
 2. Thesystem of claim 1, comprising a voltage source electrically coupled tothe charge source and the first discharge electrode, and configured tohold the charge source at a charge potential and the first dischargeelectrode at the discharge potential.
 3. The system of claim 2, whereinthe voltage source is configured to hold the first discharge electrodeat the discharge potential having a polarity opposite a polarity of thepolarized charge.
 4. The system of claim 1, comprising a voltage sourceelectrically coupled to the charge source and the first dischargeelectrode, configured to hold the charge source at a charge potential,and wherein the first discharge electrode is electrically coupled to acircuit ground in common with the voltage source.
 5. The system of claim1, wherein the first discharge electrode is positioned and configured todischarge a portion of the polarized charge when contacted by theflame-front portion of the flame.
 6. The system of claim 1, wherein thefuel nozzle comprises a charge electrode, and is configured to apply thepolarizing charge to the fuel stream as it is emitted from the nozzle.7. The system of claim 1, wherein the charge source is configured togenerate ions.
 8. The system of claim 1, wherein the first dischargeelectrode has a toroidal shape, and is positioned coaxially with thefuel nozzle and downstream therefrom, with reference to a direction offlow of a stream of fuel emitted by the nozzle.
 9. The system of claim8, wherein the fuel nozzle is configured to emit the fuel stream in adivergent cone, and wherein the toroidally-shaped first dischargeelectrode is sized and positioned such that an inner diameter of thefirst discharge electrode is at least equal to a diameter of thedivergent cone at a point at which the fuel stream passes the firstdischarge electrode.
 10. The system of claim 8, comprising an actuatorcoupled to the first discharge electrode and configured to move theelectrode relative to the nozzle.
 11. The system of claim 10, whereinthe actuator is configured to rotate the first discharge electrode aboutan axis lying perpendicular to a longitudinal axis of the nozzle. 12.The system of claim 11, wherein the actuator is configured to adjust anangular position of the first discharge electrode relative to thelongitudinal axis of the nozzle.
 13. The system of claim 10, wherein theactuator is configured to translate the first discharge electrode alonga line parallel to a longitudinal axis of the nozzle.
 14. The system ofclaim 1, comprising: a plurality of discharge electrodes, including thefirst discharge electrode; a voltage source electrically coupled to thecharge source and to each of the plurality of discharge electrodes. 15.The system of claim 14, wherein the plurality of discharge electrodesare positioned in radial symmetry around a longitudinal axis of the fuelnozzle.
 16. The system of claim 14, wherein the voltage source isconfigured to hold each of the plurality of discharge electrodes at thedischarge potential.
 17. The system of claim 16, wherein the voltagesource is configured to selectively hold some of the plurality ofdischarge electrodes at the discharge potential, while decoupling othersof the plurality of discharge electrodes from a circuit including thevoltage source and the charge source.
 18. The system of claim 15,wherein each of the plurality of discharge electrodes is positioned atone of a plurality of distances radially from the longitudinal axis ofthe fuel nozzle.
 19. The system of claim 15, wherein each of theplurality of discharge electrodes is positioned at one of a plurality ofdistances axially from the fuel nozzle.
 20. A method, comprising:emitting a fuel stream from a burner nozzle of a combustion system;applying an electrical charge to a flame supported by the fuel stream;attracting a flame front of the flame toward a discharge electrode byholding the discharge electrode at a discharge potential, relative tothe electrical charge.
 21. The method of claim 20, wherein the holdingthe discharge electrode at a discharge potential includes holding thedischarge electrode at a potential having a polarity that is opposite apolarity of the electrical charge.
 22. The method of claim 20, whereinthe holding the discharge electrode at a discharge potential includesthe discharge electrode at a ground potential that is common to acircuit coupled to charge source configured to apply the electricalcharge.
 23. The method of claim 20, wherein the attracting a flame frontof the flame toward a discharge electrode includes discharging a portionof the electrical charge upon contact of the flame front with thedischarge electrode.
 24. The method of claim 20, wherein the applying anelectrical charge to a flame includes applying the electrical charge tothe fuel stream as it exits the burner nozzle.
 25. The method of claim24, wherein the applying the electrical charge to the fuel streamincludes applying the electrical charge to an electrically conductiveportion of the burner nozzle.
 26. The method of claim 20, wherein theapplying an electrical charge to a flame includes generating ions andintroducing the ions to the flame.
 27. The method of claim 20, whereinthe holding the discharge electrode at a discharge potential includesholding a toroidally-shaped discharge electrode at the dischargepotential.
 28. The method of claim 20, wherein the holding the dischargeelectrode at a discharge potential includes holding a plurality ofdischarge electrodes at the discharge potential.